156 Inorganic Chemistry, Vol. 12, No. 1, 1973 show similar values and HBPEDA' shows a slightly larger value, due presumably to the rest of the molecule as a substituent in the 2 position of the pyridine ring. Phenanthroline shows a rather low value in view of the fact that the second ring cannot rotate out of the way during initial bonding as is the case with the others. This may suggest some type of accelerating effect due to the close proximity of the second nitrogen during initial bonding. Comparison of Formation Rates of Ni(I1) with BPEDA, TKED, EDDA, and trien. The ligands BPEDA, TKED, EDDA, and trien can all be considered as N-substituted ethylenedianrines. The position of the rate-determining step, either initial bond formation or ring closure, depends on the values of k-, and k z in eq 11. With the exception of trien, all kz values should be about the same since only coordinated nitrogens appear to have a sizable effect on nickel-water loss'o and since rotational barriers would be similar. Thus, the magnitude of the dissociation constant for the N-substituted dentate group really determines the position of the rate-determining step. The strong nickelnitrogen bonds, kNi(py) and kNmH,, compared to weak nickel-oxygen bonds, kNiO*c and coupled with the relative inaccessibility of secondary and tertiary nitrogens cause a shift in the rate-determining step from first to second bond formation as k-1 increases. BPEDA having k-l = 38 sec-' fits the mechanism well. Comparison of BPEDA with AMP and DPA. The formation reactions of Ni(I1) with both free and protonated forms of 2-aminomethylpyridine, AMP, and di(2-picoly1)amine, DPA, have been measured. Again, assuming no ICB effect to be present in the protonated cases, E, values can be calculated and are listed in Table 111. Since the monoprotonated forms of both ligands involve aliphatic nitrogen protonation, the E, values reflect steric factors for attack at
Lawrence Que, Jr., and L. H. Pignolet the aromatic nitrogens. The value for HDPA' is similar to that for HBPEDA' as would be expected but that for HAMP' is abnormally low. Calculations of E, for AMP and DPA result in anomously large values. In the case of DPA, pKHL is 2 (2) magnetic moment and pteffis the magnetic moment of the tetrahedral isomer determined, when possible, from solidstate susceptibility measurements (Tables I and 11). If pteff= 3.15 BM, the average tetrahedral value observed, good agreement between these two methods is found. Free energy values, AG, were determined using eq 1 and 3 in which AG = AG = -RT In (NJ(1 - N t ) )
(3) -RT In Ke, and Keg = Nt/(1 - N t ) . The AG values were then fit by least squares t o the equation AG = A H - TAS. AH and A S values are presented in Table 11. AG andNt values are accurate to within +30 cal/mol and k0.02, respectively. Kinetic Parameters. Kinetic results were obtained by monitoring the line widths at half-height, v1,2 (Hz), of the rnH or cyclopropyl P-H2 resonances over the entire temperature range. Standard fast- and slow-exchange approximations were In the limit of fast exchange, eq 4 was usedsi2' in which T2, t , and T2, are the transverse relaxa-
k
t:,
I
(
/
40
I
.
.
I
k
.
,
I
60 ~
5.0
I
,
.
I
,
I
,
,
, i
5.0 ~
40 '
i i io3 ~ i io3~ Figure 4. Log plots of n times the observed line widths vs. reciprocal temperature for NiL2X, complexes in CD,Cl, solution. Dashed line shows extrapolation used in kinetic analysis for Ni( P P ~ C Y P , ) ~ B. ~ ,
100:
50-
5 1
107
5-
(4) tion times for the observed averaged resonance and for the tetrahedral and planar resonances in the absence of exchange, respectively. Np is the mole fraction of planar species, Aut is the isotropic shift for the tetrahedral isomer, and rt and rp are preexchange lifetimes defined by eq 5 in which kt = (7J-l and kp = ( T ~ ) - ~ In . the limit of slow exchange, eq 6 was kt tetrahedral --f planar
6
(5 1
IF
0.5-
t t
*\
\ ' I I
I
I
I
I
4.0
IIT x
I
io3
I
I
5.0
Figure 5. Log ( k t / T )vs. 7'-' plot for NiL,X, solution.
1
#I
complexes in CD,Cl,
used for the observed line width of the tetrahedral isomer.79892s cyclopropyl P-H2 resonances. These values were estimated from the frozen-out spectra.30 This analysis yields .rt in the slow and fast-exchange regions. First-order rate constants, 4 , and activation The transverse relaxation time is related to v 1/z by l / T 2 = parameters were determined from least-squares fits to n v I l 2 . Tzt values were determined in the region of exchange In ( k t / T )vs. 1/T plots (Figure 5 ) assuming the validity of eq 7 broadening by linear interpolation between the frozen-out k - k T e x pAS* [ T - mAH* ] and completely averaged regions on In nv1,2 vs. T'l plots (7) t h shown in Figure 4 for NiBr2@PhCyp2), . T2, in the fastexchange region was calculated by Tzt = NtT2. T z pwas in which AS* and AH# are the entropy and enthalpy changes assumed constant throughout the temperature range and was of activation. Kinetic results are shown in Table IV. calculated using a 5-Hz line width for both the rn-H and The errors in AG* and kt reported at -50" are small whereas the errors in @ and AS* are significantly larger.
(28) J . A. Pople, W. G. Schneider, and H. J. Bernstein, "High Resolution Nuclear Magnetic Resonances," McGraw-Hill, New York, N. Y., 1959, Chapter 10. (29) L. H. Piette and W. A. Anderson, J. Chem. Phys., 30, 899 (1959).
(30) This procedure has been used by La Mar and Shermansb and was justified because the kinetic parameters are primarily affected by the N t / T , t term in eq 4.
Dihalobis(tertiary phosphine)nickel(II) Complexes Results and Discussion The purpose of this study is to further our understanding of the role of steric and electronic properties of some tertiary phosphine ligands by examining their effect on structural interconversion I. We have examined a number of complexes of the type NiX2(PR3)2 where X = C1 or Br and R is alkyl or phenyl. The phosphine ligands include trialkyl, dialkylphenyl, and alkyldiphenyl types where alkyl groups include cyclopropyl, cyclohexyl, methyl, vinyl, and tert-butyl. Characterization data for all complexes investigated are shown in Table I.22 The cyclopropylsubstituted complexes are new with the exception of NiC12( P C Y P ~ )which ~ , was reported by Shupack" to be the only trialkylphosphine complex exhibiting equilibrium I in solution (all others are presumably planar). We decided to study the cyclopropyl series because of this interesting result and because of the cyclopropyl group's intriguing electronic proper tie^.^' The cyclohexyl-substituted complexes were synthesized in order to examine the importance of ligand steric effects. During the course of our work Stone and Dori" reported that NiBr2(PCyh3)2 is 32%tetrahedral. This result is in disagreement with ours (vide infra). The other complexes were selected for comparison to the cyclopropyl and cyclohexyl series. We have measured the thermodynamic properties of these complexes by pmr, magnetic susceptibility, and electronic spectral experiments. Tables I1 and I11 contain the magnetic and spectral data, respectively. In the last section of the paper we report the results of a kinetic study on four of these complexes. Kinetic data are shown in Table IV. The Structural Interconversion. Pmr and Thermodynamics. The temperature dependence of the pmr shifts for NiC12(PCyp,), in CD2C12 is shown in Figure 1. Above cu. -55", the resonances are averaged over the planar and tetrahedral forms whereas below cu. - 70°, separate resonances are observed. The intermediate temperature region exhibits exchange-broadened peaks. Curie plots for the m-H or cyclogropyl P-H, 32 resonances for all complexes examined are illustrated in Figures 2 and 3. The low-temperature parts of the plots clearly show linear Curie behavior for the frozen-out tetrahedral species. In general these lines have nonzero intercepts as pointed out in the treatment of data section and are shown in Table 11. At higher temperatures, non-Curie temperature dependence which is characteristic of equilibrium I and given by eq 8 is observed. In this
equation Aviav is the observed isotropic shift of resonance i,
q is the electron-nucleus hyperfine coupling constant in gauss, B is a temperature-independent term shown in Table 11, and Nt is the mole fraction of tetrahedral species and is defined in eq 1-3. The other symbols have their usual meanings.& This behavior is observed for all complexes studied with the exception of NiBr2(PCyh3)2 and NiBr2(PPh2 - ~ - B u ) ~The . former complex showed no isotropic shifts at 3 1" which is consistent with Nt = 0 while the latter showed linear Curie temperature dependence from +3 1 to -75' which is consistent withN, = 1.0. Solution magnetic susceptibility measurements are consistent with the pmr results because moments inter( 3 1 ) A. D. Walsh, Trans. Faraday SOC.,45, 179 (1949); C. H. Heathcock and S. R. Poulter, J. Amer. Chem. SOC.,90, 3166 (1968). (32) The cpclopropyl P-H2 resonance was only used for PCyp, complexes. This resonance is labeled P-H2 in Figure 1.
Inorganic Chemistry, Vol. 12, No. 1, 1973 161 mediate between 0 and cu. 3.00 BM are found in all cases except with the above-mentioned two complexes. The solution and solid moments for NiBr2(PCyh3)?,indicate a planar diamagnetic complex. Stone and Dori" have reported that pen = 1.81 BM for this complex in CH2C12 which is in disagreement with our result. We have measured this moment on numerous occasions in several thoroughly degassed solvents and have concluded that the compound slowly reacts with CH2C12 yielding paramagnetic products. The pmr spectrum at 3 1" in CD2C12 (freshly treated with K2C03) shows the appearance of several isotropically shifted peaks which grow in with time and are quite large after cu. 30 min. A temperature-dependent study showed linear Curie dependence for all of these new peaks, which indicates that they are fully paramagnetic and that no configurational equilibrium is present.33 All of the complexes studied are subject to slow solvolysis with halogenated solvents which necessitates measurements to be made quickly on fresh solutions. The pmr and magnetic susceptibility data are only consistent with equilibrium I.798 The thermodynamic results obtained from pmr measurements are shown in Table 11. The magnetic data yield similar Nt values but are not presented became accurate tetrahedral solution moments are not known. The position of equilibrium I is best described by Nt or AG at 31" because these parameters contain the smallest experimental uncertainty. The other parameters, AH and AS, contain larger errors because they are much more sensitive to the extrapolation of isotropic shifts from frozen-out spectra. The values of AS are within experimental uncertainty but solvent effects can easily cause the observed variations. The AS values reported here are higher than those previously reported for similar compounds by cu. 2-3 eu because in the present study the thermodynamic parameters were calculated empirically, i.e., not assuming the Curie law (see treatment of data section). Other studies assumed Curie temperature Large variations in Nt are caused predominantly by variations in AH whereas small effects could result from variations in AS. Significant substituent effects (Ut values >O. 10) are therefore caused by enthalpy changes and may be best understood in terms of relative stabilization or destabilization of the planar and tetrahedral isomers. In all cases where thermodynamic parameters were measured, the planar isomer is energetically more stable than the tetrahedral. This observation is consistent with most compounds exhibiting equilibrium Ligand Field Spectra. Electronic spectral results are also consistent with equilibrium I. Bands assignable to the tetrahedral (v2 [3T1 + 3A2]) and planar adxy -+ . . .dxyd,a +) isomers are observed.7a The transition labeled v1 is actually a combination band composed of both planar and tetrahedral [3T1(F) + 3T,(P)] transitions. Evidence for the planar component of vl is obtained by examining the extinction coefficients as a function of Nt (Table 111). At low N,, for v2 is small whereas e for v1 is large. AsNt (e
(33) The variable-temperature pmr spectrum is the best instrument for demonstrating the presence of the planar-tetrahedral equilibrium because it shows the individual species causing anomalous magnetism. Stone and Dori" also reported a somewhat lower moment for this complex in benzene. Our experiments show diamagnetism and no isotropic shifts in toluene. We have noted that oxidation o f the complex results in some phosphine oxide impurity which yields a small but measurable magnetic moment. This is presumably due t o phosphine oxide coordination leading t o tetrahedral stereochemistry: F. A. Cotton and D. M. L. Goodgame, J. Amer. Chem. SOC.,8 2 , 5771 (1960).
Lawrence Que, Jr., and L. H. Pignolet
162 Inorganic Chemistry, Vol. 12, No. 1, 1973 increases, E for v2 increases while E for v1 slowly decreases but never below 190 1. mol-' cm-'. This behavior has been previously observed and is characteristic of equilibrium I. The position of v 2 reflects the ligand field strength of the tetrahedral isomer whereas v1 cannot be easily interpreted because the overlapping absorptions obscure the peak positions. Substituent Effects on Thermodynamics. The position of equilibrium I is extremely sensitive to the halide and the phosphine substituent. All studies to date including the present have shown that Nt increases in the order C1< Br < I with constant phosphine.6-s310 In this study, N i x 2 (PPh2Cyp), hasNt = 0.56 and 0.92 for X = C1 and Br, respectively. This trend has been interpreted as resulting from both steric and electronic effects. Space-filling models show steric crowding between phosphine ligands and the halides in trans-planar complexes. This strain is relieved in the tetrahedral isomer and hence the order in Nt depends on the size of the halide. Indeed the average P-Ni-Br angle in NiBr2(PPhzBz), is 92.7 and 108.2' for the planar and tetrahedral isomers, r e s p e ~ t i v e l y . ~Electronic ~ arguments are less clear because the ligand field strength follows the spectrochemical series (C1> Br > I) for both isomers. Phosphine substituents also show a marked effect on N,. For example, in this study Nt varies from 0.0 to 0.95 for NiBrzLz where L = PCyh3 and PCyp3, respectively. Steric" and e l e ~ t r o n i c 'arguments ~ have been put forth to explain trends in Nt as a function of phosphine substituent. The thermodynamic results in Table I1 yield proof that a trialkylphosphine and several dialkylphenylphosphine ligands can indeed exhibit equilibrium I in NiX2L2 type complexes. These results derive from pmr, electronic, and magnetic data. Previous reports on complexes employing these ligands were based on magnetic10320 measurements alone. The factors which determine this stereochemistry are discussed below. (1) Steric Effects. The results in Table I1 indicate that steric effects of phosphine substituents are relatively unimportant in influencing Nt . This is clearly seen in the series of NiBr2Lz complexes where L, Nt are as follows: PPh3, 1.0; PCyp,, 0.95; PCyh3, 0.0. Tricyclohexylphosphine is the largest phosphine but prefers a planar stereochemistry whereas the smaller tricyclopropylphosphine stabilizes the tetrahedral. Steric arguments would predict the opposite effect because the larger, more bulky phosphine would destabilize the crowded planar isomer. Steric effects are evident, however, as shown in the series of NiBr2(PPh,R)2 complexes where R,Nt are as follows: CH3, 0.64; C2H5,O.67fb n-C4Hgr0.70;8b tC4H9, 1.OO. The steric influence of the t-C4H9 substituent greatly destabilizes the planar isomer whereas the n-alkyl groups have only a small effect. The pmr of the tC4H9 complex shows splittings of methyl and phenyl meta resonances at 3 1" which result from hindered rotation of P-C bonds which demonstrates steric strain in the tetrahedral isomer. The strain in the planar isomer is expected to be great enough to prevent its formation. These arguments show that steric factors are only important in affecting N, in extreme cases. An interesting trend in v2 (Table 111) as a function of phosphine substituent sheds more light on ligand steric effects. In the cyclohexyl series with X = Br [L, v2 (cm-') = PPh3, 10,980; PPh2Cyh, 11,360;PPhCyh, , 11,1201 the ligand field strength should follow the trend in ligand basicity (vide infra) which explains the increase from
PPh3 to PPhzCyh. The subsequent decrease in ligand field strength on going to PPhCyh, can only be explained by steric lengthening of the Ni-P bond. This observation has been noticed by others." In the cyclopropyl series where steric strain is minimized the following trend is observed with X = Br: L, v2 (cm-')=PPh3, 10,980; PPh2Cyp, 11,520;PPhCyp2, 11,560; PCyp3, 11,840. Here we see the expected correlation with ligand basicity. Further evidence for steric weakening of the ligand field is seen in the series with X = Br and L = PPh,R: R, v 2 (cm-' ) = CH3, 11,680; CzHS, 1 1,360; n-C4H9, 11,420; t-C4H,, 10,800.34 These phosphines have similar electronic properties but different steric requirements. These steric effects should be even more important in the planar isomers but the planar transition cannot be independently observed (vide supra). (2) Electronic Effects. Electronic arguments are clearly needed to explain why NiBrz(PCyh3), is planar and NiBr, ( P C Y P ~is) ~greater than 95% tetrahedral. Indeed, electronic arguments have often been used to explain why triarylphosphines always cause tetrahedral stereochemistry, while replacement of aryl with alkyl groups stabilizes the planar (or destabilizes the tetrahedral).699a3 10920 These arguments usually take the form that effects which weaken the ligand field tend to stabilize the tetrahedral form relative to the planar form. Such arguments are vague because it is not clear to which isomer the ligand field strength refers. It has recently been shown that phenyl para substituents, which are electron withdrawing, decrease v2 but increase v 1 (planar).7a These same electron-withdrawing groups were found to stabilize the planar isomer relative to the tetrahedral.7a These observations contradict earlier explanations. In the present work the tetrahedral ligand field strength in N i B r , ( P c ~ p ~is) ~the strongest we have observed, yet Nt > 0.95. Clearly, electronic effects are not as simple as have been argued. The relative u-donor strength of phosphine ligands can be determined by measurements of basicity. Streuli3' has determined numerous pK, values of substituted phosphines and Denney and Gross have recently measured the pK, of P C Y P ~ .Pertinent ~~ results are as follows for L, p ~ , : PPh3, 2.73; PPhMe2, 6.49; PCyp3, 7.60; P-i-Bu3,7.97; PMe3, 8.65; PCyh3, 9.70. These data illustrate that PPh3 is a much weaker base than any trialkylphosphines. The low ligand field strength in NiBrz(PPh3)2 clearly results from this weak u-donor property. However, this cannot be the only factor which causes high Nt because PCyp3 which has a basicity similar to other trialkylphosphines produces very large ligand fields and yields high values of Nt. The electronic interaction which causes high Nt values must involve a n interaction among phosphorus, nickel, and substituents. Convincing evidence for n interaction in an inductive sense has been r e p ~ r t e d . ' ~Thermodynamic and electronic data were measured for NiX2(PMe(aryl)2)z type complexes where phenyl para substituents were varied over a wide range of Hammett up values. A linear correlation between AG and Xup was found such that the more electron-withdrawing para substituents (high XuD) caused high AG values (low N,). A corresponding trend was observed with v,(tetrahedral) and vl(planar). Groups which caused high Nt (34) Table 111 and ref 9a. (35) C. A. Streuli, Anal. Chem., 32, 985 (1960); W.A. Henderson, Jr., and C. A. Streuli, J. Amer. Chem. SOC.,8 2 , 5191 (1960). (36) (1967).
D. B. Denney and F. J . Gross, J. Org. Chem., 32, 2445
Dihalobis(tertiary phosphine)nickel(II) Complexes values resulted in high v2 and low v l . Linear correlations of this type were found. These results were interpreted in terms of ligand field stabilization energy (LFSE). The decrease in vl(planar) with increasing electron-withdrawing ability was interpreted as a n-bonding interaction (see Figure 7 of ref 7a). This argument cannot account for the differences in PPh3, PCyp3, and PCyh3. Phenyl and cyclopropyl groups have n-symmetry orbitals which can conjugate to the phosphorus 3d orbitals. The cyclohexyl group cannot participate in this type of interaction. There are numerous results which support nbonding and conjugative effects produced by the cyclopropyl g r o ~ p . ~ This ~ , conjugative ~ ~ $ ~ ~ interaction will decrease the ligand field splitting in the planar isomer in the following way. Overlap of the n-symmetry orbitals of the phenyl or cyclopropyl group with the 3d orbitals of phosphorus will reduce the n-acceptor capacity of the phosphorus atom. This will reduce the nickel d,, - d,, separation by destabilizing the r-symmetry d,. ~ r b i t a l . ~ " The d,, +,orbital has u symmetry and will not be affected by this interaction. The ligand field splitting in the tetrahedral isomer will not show this large n effect because the tz and e orbitals both have n symmetry and will be similarly a f f e ~ t e d . ~ 'Thus, these groups will reduce the LFSE of the planar isomer relative to the tetrahedral. The planar transition, v 1 , cannot be accurately measured in these complexes because of interference from the 3T1 3T1(P) tetrahedral transition. Stone and Dori" have examined a series of planar thiocyanate complexes of the type Ni(NCS),L2 in which vl(planar) decreases in the order PCyh3 > PPhCyh, > PPhzCyh > PPh3. The above arguments provide evidence for n interaction between cyclopropyl and phosphorus and between phosphorus and nickel. The vinyl group is also expected to n bond with the phosphorus and thereby destabilize the planar isomer. The low Nt value presumably results because PPhzVy produces a weaker tetrahedral ligand field than PPh2Cyp (v, in Table 111). Therefore the LFSE of the tetrahedral isomer is also lowered resulting in the observed Nt. This illustrates that the relative Nt values for these complexes are a sensitive function of competitive electronic effects in both isomers. Substituent Effects on Kinetic Parameters. Kinetic parameters for five complexes are reported in Table IV. aS*values are all small and AH* values reflect the trends in rate constant. The parameters are similar to those reported by La Mar and Shermansb for analogous complexes. The most a$curate parameters for comparison are AG*(- 50') or kC5O because these values were determined in the region of line broadening and are not subject to extrapolation errors. The halide dependence, C1 faster than Br, is consistent with other investigation^.^'^ The dependence on phosphine substituent is less obvious and, as concluded by other^,^^,^^ uninterpretable. The results for --f
(37) M. J. Jorgenson and T. Leung, J. Amer. Chem. SOC.,90, 3769 (1968);G.Dauben and G. H. Berezin, ibid., 89, 3449 (1967). (38) F. A. Van Catledge, submitted for publication. (39) This argument is essentially the one put forth by Venanzi to explain why the replacement of alkyl groups by phenyl groups in Ni(PR,),X, complexes weakens the planar crystal field energy (see Figure 5 of C. R. C. Cousmaker, M. H. Hutchinson, J. R. Mellor, L. E. Sutton, and L. M. Venanzi, J. Chem. SOC.,2705 (1961)). The observations of Pignolet, Horrocks, and Holm" are also consistent with this description. Inductive n perturbations will further vary the LFSE of the planar and tetrahedral isomers according to Figure 7 of ref 7a and arguments therein. (40) La Mar has shown that n bonding is unimportant in tetrahedral dihalobis(tertiary phosphine)nickel(II) complexes: G . N. La Mar, E. 0. Sherman, and G. A. Fuchs, J. Coord. Chem., 1, 289 (1971).
Inorganic Chemistry, Vol. 12,No. 1, 1973 163 PCyp, and PPhCyp, ligands are the first reported for trialkyl- and aryldialkylphosphines. However, these ligands cause no unusual kinetic effects. La Mar and Shermansb reported a second-order kinetic acceleration of the rate of planar-tetrahedral interconversion for NiBrz(F'PhzMe)z with added phosphine ligand. We have performed similar experiments with NiBrz(PPhzCyh), and excess PPh2Cyh with completely different results. In this case, as much as 0.28 molar excess ligand failed to perturb the kinetics of interconversion. The free ligand appeared in the pmr spectrum at its unshifted diamagnetic position even at +80°. In the case of PPh2Me, as little as 0.0003 M excess phosphine significantly accelerated the rate. This kinetic acceleration presumably results from a ligand exchange process because the added ligand resonances are averaged with the planar and tetrahedral resonan~es.~' La Mar and Shermansb pointed out that PPh,Me must exchange with its tetrahedral complex faster than PPh3 with NiBr2(PPh3),. Ligand-exchange kinetics of the latter have been measured and are second order in added p h ~ s p h i n e . ~ 'Our results show that PPhzCyh ligand exchange is even slower. This must result from steric hindrance caused by the bulkier PPhzCyh ligand which would slow down second-order ligand exchange. T 0 1 m a n ~has ~ shown that steric factors are more important than electronic factors in determining the thermodynamics and kinetics of ligand exchange in NiL4 type complexes. The relative sizes of tertiary phosphine ligands can be estimated from their steric cones defined by T 0 1 m a n . ~ ~This order is PCyh3 > PPh3 > PMe3 which supports our observations. Registry No. sp-NiClz(PCyp3),, 36673-22-0;sp-NiCl, (PPhCyp,), ,36673-24-2; sp-NiC12(PPhzCyp), ,36655-08-0; sp-NiBr,(PPhCypz),, 36673-25-3;sp-NiBrz(PCyh3),, 3667326-4; ~p-NiBr,(PPhCyh~)~, 36673-27-5; sp-NiBr2(PPhzCyh),, 36673-28-6;sp-NiBrz(PPh2Vy), ,36673-29-7;sp-NiBr, (PPh,Me), ,28582-49-2; td-NiC12(PCyp3),, 36673-23-1;tdNiClz(PPhCyp2), ,36673-31-1; td-NiClz(PPhzCyp)z, 36655)~, td-NiBrz(PPhCypz),, 09-1 ;t d - N i B r , ( P C ~ p ~36673-32-2; 36688-77-4; td-NiBr2(PPhzCyp), ,36655-10-4; td-NiBr, (PPh2Cyh),, 36655-1 1-5; td-NiBrz(PPh2Vy),, 36673-33-3; td-NiBrz(PPh,-t-Bu),, 36673-34-4;td-NiBr2(PPhzMe),, 36673-35-5;td-NiBr2(PPh&, 36673-36-6. Acknowledgments. This research was supported by grants from the Petroleum Research Fund administered by the American Chemical Society, the Research Corp., and the University of Minnesota Graduate School. L. Q. is a recipient of the Lubrizol Fellowship administered by the Department of Chemistry. We also acknowledge an NSF instrument grant for purchase of a Varian XL- 100-15 nmr spectrometer. We thank Dr. G. N. La Mar for several useful discussions. (41) We have noticed that added PPh,Me not only accelerates the rate of the planar-tetrahedral interconversion but also changes the equilibrium constant. The added phosphine favors formation of the planar isomer requiring kt+p t o be increased over kp*. This implies that the role of added phosphine is to produce a five-coordinate transition state primarily from the tetrahedral isomer which would rapidly convert into the planar and tetrahedral isomers. The net result, provided ligand exchange is faster than the planar-tetrahedral interconversion, would be t o accelerate the interconversion and shift the equilibrium toward the planar form. (42) L. H. Pignolet and W. D. Horrocks, Jr., J. Amer. Chem. SOC.,90,922 (1968). (43) C. A. Tolman, J. Amer. Chem. SOC., 92,2956 (1970).
